The present invention relates to a process for a high-temperature catalytic chemical reaction which uses a solid catalyst comprising an active metal phase and a non-oxidized refractory catalytic support on which the active phase is deposited.
More particularly, the catalytic support is based on silicon nitride (Si3N4).
Silicon nitride exists in two crystalline forms, the xcex1 form (xcex1-Si3N4), known as the low temperature form ( less than 1200xc2x0 C.), and the xcex2 form (xcex2-Si3N4), known as the high temperature form ( greater than 1500xc2x0 C.). The two forms coexist between 1200xc2x0 C. and 1500xc2x0 C. The structures of each of these forms are essentially distinguished by a different arrangement of the SiN4 tetrahedra.
V.I. Simagina et al. [1] have described the use, for the oxidation reaction of methane, of a catalyst comprising, as catalytic support, porous xcex2-Si3N4 on which nickel oxide is dispersed. In order to obtain a satisfactory conversion yield, the reaction temperature has to reach 850xc2x0 C. This is because the stability of the methane molecule requires high temperatures for initiating the reaction. However, the use of high temperatures results in the undesirable production of contaminants, such as carbon monoxide, resulting from the decomposition of carbon dioxide and/or from the partial oxidation of methane, and nitrogen oxides, originating from the oxidation of atmospheric nitrogen.
The authors of the present invention have looked for a catalyst which is sufficiently active to catalyze reactions of the abovementioned type but which makes it possible to lower the temperature thereof in order to eliminate the abovementioned disadvantages while having high stability and high reactivity under particularly severe conditions, namely oxidizing atmosphere and high exothermicity.
Thus, they have discovered, for catalytic chemical reactions which normally take place at high temperature, that a solid catalyst comprising an active metal phase and a refractory catalytic support comprising mainly silicon nitride in its xcex1 form makes it possible to lower the reaction temperature to and to control it at a value of between 200xc2x0 C. and 800xc2x0 C. and preferably between 200xc2x0 C. and 600xc2x0 C.
The silicon nitride is advantageously in the substantially non-porous state.
Before describing the present invention in more detail, certain terms employed are defined hereinbelow.
xe2x80x9cReaction which normally takes place at high temperaturexe2x80x9d comprises reactions which only take place under or the yield of which is improved only under high temperature conditions (so-called xe2x80x9coperating temperaturexe2x80x9d) and also highly exothermic reactions, the operating temperature of which is not necessarily very high but which release thermal energy which raises the local operating temperature with the possible creation of hot spots.
xe2x80x9cSubstantially non-porousxe2x80x9d is understood to mean a material which does not exhibit a hysteresis cycle in nitrogen adsorption/desorption isotherms at the temperature of liquid nitrogen.
According to the invention, the support comprises mainly silicon nitride in its xcex1 form; this means that it is composed of 90% at least of said form, it being possible for the remainder to be in particular the xcex2 form of silicon nitride.
The preferred alternative forms of the process of the invention are presented hereinbelow:
the support is advantageously in the discrete form with a particle size of between 0.1 and 1 xcexcm;
the active metal phase, that is to say the catalytic phase, is chosen in particular from transition metals and preferably consists of palladium;
the process of the invention is particularly suitable for the catalysis of chemical reactions which take place in an oxidizing atmosphere, such as oxidations; thus, the combustion reaction of methane, in an oxidizing atmosphere, which is illustrated in the examples which will follow, is advantageously carried out according to the process described hereinabove,
the support exhibits a specific surface preferably of between 5 and 20 m2/g,
the ratio of the proportion by weight of the metal of the active metal phase and in particular of palladium to that of the silicon of the support is advantageously between 0.5 and 2% by weight,
the catalyst is preferably obtained by impregnating the support with metal acetylacetonate salts and then activating according to a protocol which will be described later.
Another subject-matter of the invention is a catalyst comprising an active metal phase and a refractory catalytic support comprising mainly silicon nitride in its xcex1 form. Preferably, the support is substantially non-porous.